Chemical sensor using chemically induced electron-hole production at a schottky barrier

ABSTRACT

Electro-hole production at a Schottky barrier has recently been observed experimentally as a result of chemical processes. This conversion of chemical energy to electronic energy may serve as a basic link between chemistry and electronics and offers the potential for generation of unique electronic signatures for chemical reactions and the creation of a new class of solide state chemical sensors. Detection of the following chemical species was established: hydrogen, deuterion, carbon monoxide, molecular oxygen. The detector ( 1   b ) consists of a Schottky diode between an Si layer and an ultrathin metal layer with zero force electrical contacts.

This application is a divisional application of application Ser. No.10/846,433 filed May 14, 2004, currently pending; which is acontinuation application of application Ser. No. 10/170,000 filed Jul.11, 2002, currently pending, which is the U.S. national phase ofPCT/US99/29363 filed Jan. 19, 2000.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The field endeavor of the invention relates to sensors for detectingchemicals and in particular to a sensor for detecting and distinguishingatomic hydrogen or atomic deuterium oxygen, carbon monoxide, and nitricoxide.

2. Description of the Prior Art

Electron transport through a metal-semiconductor interface is determinedlargely by the Schottky barrier between them.

The detailed pathways of energy transfer in exothermic and endothermicreactions at metal surface is incompletely understood and of fundamentalinterest. Bond formation energy of up to several electron volts istransferred into the substrate during such exothermic reactions. Sincebulk phonon energies are typically two orders of magnitude smaller, ithas been appreciated by the prior art that non-adiabatic excitations ofelectron-hole pairs may be an alternative to the creation of multiplephonons as a mechanism for sensor detectors. With surface reactions atthermal collision energies, there are few examples of energytransferring to the electronic system accompanied by light emission orchemiluminescence and exoelectron ejection. Chemiluminescence andexoelectron injection are observed only with exothermic adsorption ofelectronegative molecules on reactive metal surfaces. In addition,exoelectron emission requires that the metal have a low work function.Heretofore, there has been no direct experimental evidence foradsorption induced electron-hole pair excitations at transition metalsurfaces.

Therefore, what is needed is some type of sensor design or principal inwhich adsorption induced electron hole pair excitations at a transitionmetal surface can be exploited to provide a chemical sensor.

BRIEF SUMMARY OF THE INVENTION

The invention is a silicon device structure, or more specifically ametal-semiconductor Schottky diode, which exploits the current-voltagecharacteristics of the diode for separation of charge and theinteraction of the surface adsorbates on the metal to produce electronsor holes of sufficient energy to transverse the ultrathin metal film andcross the Schottky barrier. The structure allows reliable, zero forceelectrical contacts to be made to metal films less than 100 Angstromsthick. In one embodiment two metalized contacts are deposited usingphotolithographic techniques on a 4000 Angstrom oxide layer prepared onSi (111). The oxide is etched from between the contacts and the exposed6 mm×6 mm Si (111) surface is wet chemically treated. Under vacuumconditions ultrathin metal is deposited onto the device to form a diodeunder well defined conditions.

The sensor device may be microfabricated on n- or p-doped semiconductorwafers. In the illustrated embodiment p_(n)=5-10 Ωcm, p_(p)=1-20 Ωcm),in an ohmic contact is provided on the back of wafer by means of by As⁺and B⁺ ion implantation, respectively. Isolated from the silicon, thethick gold contact pads are evaporated on a 4000 angstrom thermal oxidelayer on the opposing or front side of the device. A 0.3 cm² window ischemically wet etched through the oxide layer between isolated the goldpads through the use of buffered hydrofluoric acid leaving a clean,passivated silicon surface. The device is then transferred into anultrahigh vacuum chamber (p≈10⁻⁸ Pa) for metal deposition andmeasurement.

Copper and silver films, for example, are deposited by e-beamevaporation at substrate temperatures of 135° K. The nominal thicknessis measured by a quartz microbalance. The etching of the oxide producesan angle of inclination between the oxide and the top surface of thesilicon substrate with typically 25°. The evaporated thin metal filmsare connected to the thick gold pads across the small inclination angleto provide a zero force front contact to the device. This contact designallows electrical contact for the current/voltage measurements betweenthe front contacts and back contact even with film thicknesses below 80angstroms.

In preliminary experiments investigating the energy transfer duringchemisorption, a new process has been discovered associated withchemisorption of atomic hydrogen or atomic deuterium on Ag and Cuultrathin films. When these metals are deposited (30 Angstroms −150Angstroms) onto Si(111) in a Schottky diode detector structure, acurrent is generated associated with an incident atomic H or D beam onthe film. It is hypothesized that this “chemicurrent” is a result ofchemisorption induced excited charge carriers which traverse theSchottky barrier. That energy transfer from chemisorption can proceed bydirect electronic excitation is a significant departure from theconventional dogma which holds that multiple phonon excitation is themeans through which the heat of adsorption is dissipated.

The implications of this observation for the study of surface catalyzedreactions are many. In addition, this process serves as a basic linkbetween chemical processes and electronics and offers the potential forthe generation of unique electronic signatures for chemical reactionsand the creation of a new class of solid-state chemical sensor. Thefirst direct means of measuring atomic H or atomic D separate from thediatomic molecule is demonstrated below. More importantly, it may alsobe possible to differentiate H from D on the basis of the signal. It isexpected that there are unique chemicurrent signals associated with manytypes of surface reactions.

Hot electrons and holes created at a transition metal surface, such as asilver or copper surface by adsorption of thermal hydrogen and deuteriumatoms can be measured directly with ultrathin-metal film Schottky diodedetectors on silicon (111) according to the invention. When the metalsurface is exposed to these atoms, charge carriers at the surface andtravel ballistically toward the interface. The charge carriers aredetected as a chemicurrent in the diode. The current decreases withincreasing exposure and eventually reaches a constant value at a steadystate response. The invention uses the first discovery of anon-adiabatic energy dissipation during adsorption at a transition metalsurface as a means of providing a chemical sensor or thin film “nose”able to sniff out the presence of chemicals.

The mechanism of the invention is based on the speculation that althoughthe maximum energy of any hot charge carriers are smaller than the metalwork function of the transition metal surface thereby precludingexoelectron emission, the energy of the hot-charged carriers may besufficiently large enough to enable the charge carriers to be collectedby crossing a smaller potential barrier. As will be described below thedirect detection of chemisorption-induced electron-hole pairs isfeasible using a Schottky barrier by transition metal-semiconductordiode detector. The invention shall be described in terms of an atomichydrogen adsorption on copper and silver film surfaces, however, it isto be expressly understood that many other chemical molecules orelements may be detectable on these and other different thin film metalsurfaces according to the teachings of the invention. Silver and copperfilm surfaces exhibit high reactivity to atomic hydrogen, but negligibledissassociative adsorption of molecular hydrogen, H₂. The formationenergy of the hydrogen-metal bond is large, about 2.5 electron volts inboth cases. To detect the hot charged carriers, a sensor is providedwhich is comprised of a large area of metal-semiconductor contact withan ultrathin metal film.

The device structure allows current-voltage curves to be measured fromwhich Schottky barrier heights and ideality factors as a function ofmetal film thickness can be determined. It is observed that barrierheights increases and ideality factors decreases with increasing metalfilm thickness (10 Angstroms to 100 Angstroms). Room temperatureannealing of diodes produced with a low temperature metalizationincreases the measured barrier heights and lowers the ideality factors.The magnitude of these effects depends on the metal used. Results foriron and copper on silicon (111) substrates are among the embodimentsdescribed below.

The rectifying properties of the Schottky diode formed are improved byannealing the devices to room temperature and cooling back to 135° K.The measured I-V curves can then be analyzed using thermionic emissiontheory. Effective barrier heights of 0.6-0.65 electron volts and0.5-0.55 electron volts were determined for copper and silver films of75 angstrom thickness on n-silicon (111), respectively. On p-silicon(111), silver and copper diodes showed barriers of 0.5-0.6 electronvolts. Ideality factors between 1.05 and 1.5 indicate that large-areadiodes are laterally nonuniform and exhibit a barrier heightdistribution.

The invention now having been briefly summarized turn to the followingdrawings wherein like elements are referenced by like numerals.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1(a) is a Fermi diagram of the chemicurrent detection. Hydrogenatoms react with the metal surface creating electron-hole pairs. The hotelectrons travel ballistically through the film into the semiconductorwhere they are detected.

FIG. 1(b) is a schematic side cross-sectional view through a hydrogensensing Schottky diode made according to the invention as described bythe Fermi diagram of FIG. 1(a). The ultrathin metal film is connected tothe gold pad during evaporation.

FIG. 1(c) is a plan elevational view of the device of FIGS. 1(a) and1(b).

FIGS. 2(a) and 2(b) are graphs of the chemicurrent as a function ofhydrogen exposure time for diodes with thin silver and copper filmsrespectively in a device shown in FIGS. 1(a) and 1(b). The transientscorrespond to the filling of empty adsorption sites by atomic hydrogenon the metal surfaces. The steady-state currents are explained by abalance of abstraction and re-adsorption of atomic hydrogen.

FIG. 3 is a graph of the chemicurrent, I, as a function of time, t,recorded from silver/n-Si (I 11) diodes of the type shown in FIGS. 1(a)and 1(b) exposed to atomic hydrogen and deuterium. The chemicurrent dueto atomic hydrogen adsorption is multiplied by a factor of 0.3.

FIG. 4 is a graph of the chemisorption current for a 60 Angstrom Ag/Si(111) sensor at 135K as a function of the time of exposure to CO.

FIG. 5 is a graph of the chemisorption current for an 80 Angstrom Ag/Si(111) sensor at 135K as a function of the time of exposure to CO.

FIG. 6 is a diagrammatic side view of a sensor used for catalyticchemisorption detection.

FIG. 7 is an array of sensors of the type shown in FIG. 10 in which eachone of the sensors has a different catalytic layer so the correspondingsensor detects a different reactant.

FIG. 8 is a graph of the chemisorption current for molecular oxygen on a75 Angstrom Ag/Si (111) sensor at 130K as a function of the time ofexposure to O₂.

The invention now having been illustrated in the foregoing drawing theinvention and its various embodiments now may be understood in contextin the following detailed description.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Electron-hole production at a Schottky barrier has recently beenobserved experimentally as a result of chemical processes. Thisconversion of chemical energy to electronic energy may serve as a basiclink between chemistry and electronics and offers the potential for thegeneration of unique electronic signatures for chemical reactions andthe creation of a new class of solid state chemical sensors. The initialresults have been for a atomic and molecular adsorption, however, italso expected that bimolecular surface catalyzed reactions may alsocause direct excitation of charge carriers during the formation of bondsbetween surface adsorbed species. Therefore, in addition to thedemonstrated detection of hydrogen, deuterium and oxygen, sensitivityfor chemisoprtion for carbon monoxide, carbon dioxide, molecular andatomic oxygen, molecular and atomic nitrogen, nitrogen monoxide andorganic hydrocarbons and other species is expected. Detector responsesto surface catalyzed reactions of several different combinations ofthese species following adsorption are expected to produce achemicurrent including reactions with the combinations of carbonmonoxide and molecular oxygen, carbon monoxide with nitrogen oxide andmolecular hydrogen and oxygen. The basic configuration of the detectorcan be extended to include selective coatings, mult-juncition arrays,and tunnel junctions.

The mechanism of current production in a sensor is best illustrated inFIG. 1(a) in the case of hot electrons. FIG. 1(a) is an energy diagramof a charge carriers across the metal film to silicon interface with theposition in the interface being shown on the horizontal axis and energyon the vertical axis. FIG. 1(b) is a corresponding side cross-sectionalview in an enlarged scale of the junction which is graft in FIG. 1(a).FIG. 1(c) is a plan elevational view of the device of FIGS. 1(a) and1(b). Transition metal film 10 is evaporated on an n-type silicon 12forming a diode at their interface 14 with a Schottky barrier Φillustrated in FIG. 1(a). FIG. 1(a) shows the Fermi level, EFI alsodenoted by reference numeral 16, the conduction band minimum, denoted byreference numeral 18 and the valence band minimum denoted by referencenumeral 20. If the exothermic chemisorption of hydrogen atoms createselectron-hole pairs, hot electrons may travel ballistically through film10 and across the potential barrier of the Schottky diode Φ. Theelectrodes can be detected as a current which is defined as the“chemicurrent.” Similarly, hot holes may be measured with a p-typeelectrode as well as an n-type as shown in the illustration of FIGS.1(a) and (b). The charge carrier energies lie between the barrier heightand the adsorption energy, i.e., between 0.5 and 2.5 electron voltsabove E_(F) 16. The mean free path (mfp) of electrons and holes in thisenergy range is typically on the order of 100 angstrom, as determined bythermal and field emission, internal photoemission and ballisticelectron emission microscopy. The film thickness is in the range of themean free path of the charge carriers (electrons or holes).

A silicon based device 22 was developed to facilitate contactingextremely thin metalization layers 10 during the initial Schottkybarrier formation. Devices 22 were prepared on silicon (111) substrates12 and processed using conventional silicon microfabrication techniquesto produce the device depicted in FIGS. 1(a), 1(b) and 1(c).Microfabricated substrates 12 were made from 3″ diameter 5 Ω-cmphosphorous doped n-type Si (111) wafers. Before processing, the waferswere backside ion implanted with 10¹⁵ cm⁻² zarsenic at 150 keV. Afterimplantation the wafers were diced into rectangular samples 0.45-x0.70″.The samples were then cleaned by sonication in water, acetone andisopropanol and were wet oxidized in a tube furnace to grow between 3000and 4000 Angstrom thermal oxide. For the processing of the substrates,AZ5214 image reversal photoresist was used as a positive resist. Twophotolithographic masks were used, one for front metal pads 26 and onefor oxide window 30 between pads 26. The first step of the processingwas to metalize front contact pads 26. An oxidized substrate 12 was spincoated with photoresist and patterned using a UV mask aligner. Metalpads 26 were deposited in a thermal evaporator using an initial adhesionlayer of 100 Angstroms chromium followed by 2000 Angstroms of gold.After metalization, the excess metal was removed in an isopropanolsonication lift-off. This completed front contacts 26 and the next stepwas to make back ohmic contacts 24. The ion implantation was activatedduring the thermal oxidation so that under backside oxide 25 of siliconsubstrate 12 was n+. Front side 32 of substrate 12 was coated with aprotective photoresist layer and backside oxide 25 was removed withbuffered hydrofluoric acid, HF. The backside metalization was donethrough an aluminum shadow mask. Back contacts 24 were Cr (100Angstroms)/Au (3000 Angstroms) deposited in a thermal evaporator. Tocomplete the backside metalization, the frontside photoresist wasremoved in an isopropanol sonication. The final step of the processingwas to etch a window 30 in the SiO₂ layer 28 between front contact pads26. The sample was recoated with photoresist and patterned with the maskaligner. The photoresist was developed and the exposed oxide region wasremoved with a six minute buffered HF dip. After this step thephotoresist was removed by 85° C. H₂O₂: H₂SO₄ solution. The sample wassubsequently cleaned and chemically oxidized in a fresh H₂O₂: H₂SO₄solution at 110° C. The final step was to prepare the silicon surface.After removal from the sulfuric acid, the sample was dipped in bufferedHF for 15 seconds, which was just long enough to ensure removal of thechemical oxide off the silicon surface. The sample was finally rinsed indeionized water and blown dry with nitrogen to complete the processing.Because of the etching properties of the buffered HF solution and thephotoresist, the resultant oxide had a gentle slope of 15-20 degreesfrom the unetched SiO₂ down to the silicon substrate. This angle wasindependently measured by a scanning electron microscope (SEM) and anatomic force microscope (AFM). Sloping oxide sidewall 34 allows thinSchottky metalization layer 10 to connect continuously from one goldcontact pad 26 to the other.

After the final buffered HF dip to prepare a hydrogen terminated andpassivated surface, microfabricated substrate 12 was quickly indiumbonded to a molybdenum sample holder and loaded into an ultrahigh vacuumchamber onto a sample manipulator (not shown). The manipulator has fourindependent electrical contacts, two front and two back contacts. Thetwo front contacts can be actuated from outside the vacuum chamber andwere used to electrically contact gold pads 26 on the right and leftsides of substrate 12 while the back contacts 24 contact the molybdenumsample holder. After a sample was in place on the manipulator, it waschecked for contact-to-contact current leakage. All samples used forexperiments had room temperature left-front-contact toright-front-contact resistance greater than 100 MΩ and front-to-backresistances greater than 10 MΩ.

Metal films 10 were evaporated by shuttered electron-beam wireevaporators. The evaporation rate depended on the metal used. In theembodiment where iron was used, iron was evaporated at 10 watts with arate of 10 Angstrom min⁻¹, and copper was evaporated with a heatingpower of 16 watts and a rate of 1.5 Angstrom min⁻¹. The evaporatorproduced a collimated flux that deposited a rectangular area of metalonto substrate 12. Evaporated metal film 10 spanned metal contact pads26 on either side of substrate 12, but did not extend out to the edge ofsubstrate 12.

Diodes were made on room temperate substrates as well as substratescooled to ˜130K with liquid nitrogen. A Labview virtual instrument (VI)was used to automate current-voltage measurements. The voltage sourcewas a digital-to-analog board controlled by the computer and the currentwas measured with a Kiethley picoammeter under GPIB control from thecomputer.

In the present demonstration of device 22, device 22 was maintained at135° K. and exposed to a modulated, thermal hydrogen beam produced by amicrowave plasma. Photons are extracted from the beam to avoidphotoexcitation which can be orders of magnitude stronger than thechemicurrent. A light blocking fixture was developed for the plasma tubewhich prevents photon transmission and thermalized the beam particles asis described in H. Nienhaus, et al., “______”, (to be published). Thecontent of atomic hydrogen, C_(H), i.e. the number of atoms relative tothe total number of particles in the beam, was measured with an in-linemass spectrometer. It varied typically between 7-25% where thevariations were associated with the plasma fluctuations. The kineticenergy of the atomic hydrogen was also measured between 300 and 350 K.The total atomic and molecular hydrogen impinging upon diode 22 wasapproximately constant, e.g., about 1013 particles per second. Hence,with a sensor area of 0.3 cm², the atomic flux varied between 3 and10×10¹² hydrogen atoms per cm²-second. The reaction-induced chemicurrentwas detected between the front contact 26 and back contact 24 usingstandard lock-in techniques. No bias was applied to detector 22 duringmeasurement. Due to the low temperature, the noise level was less than0.5 picoamps.

Detector current responses as a function of time of device 22 inresponse to atomic hydrogen are shown in FIG. 2 in which thechemicurrent is mapped against exposure times. FIG. 2(a) is a graphshowing the chemicurrent in silver/n-silicon, interface and asilver/p-silicon interface. FIG. 2(b) shows the chemicurrent for acopper/n-silicon diode. The atomic impingement rate, qH, was7.5±2.5×10¹¹ atoms per second. At t=0, the beam shutter was opened.Current increases instantaneously upon exposure and decaysexponentially, and eventually reaches a steady state of value as shownin FIGS. 2(a) and 2(b) at each of the diode embodiments. The dipobserved in the l/t curve for copper at about 2,000 seconds is due to adecrease in atomic hydrogen flux due to plasma instabilities. The atomichydrogen content, CH, decreases from 15% at t=1600 seconds to below thedetection limit of 2% at t=2,100 seconds in FIG. 2(b). The chemicurrentthen recovers its original value. The total beam intensity, atomic andmolecular hydrogen remained constant during this time. Thus,chemicurrent is only detected if atomic hydrogen is present. FIG. 2(a)shows that chemicurrents were detected for both p- and n-typesilver/silicon diodes, thereby implying that both hot electrons and hotholes are created by the reaction.

The chemicurrent transient shown in FIGS. 2(a) and 2(b) represents theoccupation of empty adsorption sites by atomic hydrogen on metal film10. The hydrogen coverage, Θ, increases and the adsorption probabilitydecreases with the decreasing availability of empty sites. Thesteady-state chemicurrent observed in the long time limit in FIGS. 2(a)and 2(b) is a consequence of a balance between hydrogen removal from thesurface by abstraction and re-adsorption. The chemicurrent, I, isexpected to be proportional to the hydrogen atom flux and the fractionof unoccupied adsorption sites, i.e., I=αq_(H)(Θ_(s′)−Θ), where Θ_(s) isthe saturation coverage if no abstraction occurs and α is a constant.

If the adsorption of atomic hydrogen and its abstraction by atomichydrogen in the gas phase are governed by the Langmuirian and anEley-Rideal mechanism, respectively, the time rate equation for I and Θobey a first-order kinetics described by:dI/dt∝−dΘ/dt=−(q _(H) /A)[σ_(a)(Θ_(s), −Θ)−σ_(t)Θ],where A is the active diode area, and σ_(a) and σ_(t) are the crosssections for adsorption and abstraction, respectively. By consideringthe time limits for t=0 and t→infinity, the ratio of the cross sectionsmay be determined from the maximum value, I_(max), and the steady statevalue, I_(s) of the chemicurrent via σ_(a)/σ_(r)=I_(s)/(I_(max)−I_(s)).Cross section ratio is calculated from the data in FIGS. 2(a) and 2(b)are 0.2 for the silver/n-silicon diode and 0.4 for the two other diodes.Equation (1) above predicts an exponential decay of the chemicurrentwith a time constant of A/q_(H)(σ_(a)+σ_(t)). Single exponential fits tothe data in FIG. 2 result in decay constants of 480 seconds for thesilver/p-silicon diode, 670 seconds for the copper/n-silicon diode, and750 seconds for silver/n-silicon diode. The observed variation is withinthe range of uncertainties of the beam flux. The cross section ratio anddecay constant allow the calculation of an absolute cross section if theactive diode area, A, and the hydrogen atom rate, q_(H) are known. Withan active area A=to about 0.3 cm², the analysis gives values for σ_(a)of approximately 5×10⁻¹⁶ cm² for silver and 4×10⁻¹⁶ cm² for the copperfilm. With assumed initial adsorption probability of unity, thereciprocal of the cross section σ_(a) is equal to the adsorption sitedensity. From the data, in FIGS. 2(a) and 2(b), the adsorption sitedensities of 2-3×10¹⁵ cm⁻² for both, silver and copper films obtained.These values are in excellent agreement with the number of metal atomsper unit area which is about 2.4×10¹⁵ cm² for silver on (111 silicon)and 3.1×10¹⁵ cm² copper on silicon (111) surfaces. This data supportsthe interpretation of the l/t curves of FIGS. 2(a) and 2(b) given above.Furthermore, the data shows a new way of measuring selfabstraction rateswith only one atomic species. In the prior art, the abstraction ofatomic hydrogen at surfaces is studied by deuterium-hydrogen orhydrogen-deuterium exchange reactions.

The chemicurrent is attenuated exponentially with increasing metalthickness in the silver/n-silicon diodes. The attenuation lengthcorrelates well with the mfp of electrons in silver, which furthersupports the idea that the charge carriers are created at the metalsurface and travel ballistically through the metal film intosemiconductor 12.

The quantitative difference between the n-type silver and copperSchottky diodes shown in FIGS. 2(a) and 2(b) is striking. Thesensitivity may be defined by dividing the initial chemicurrent at t=0by the hydrogen atom impingement rate. This gives the number ofelectrons detected per adsorption event as 4.5×10⁻³ for silver and1.5×10⁻⁴ for copper, an order of magnitude difference. On p-type diodes,the same sensitivity ratio is found. Thus, the difference does notcorrespond to a barrier height difference, which is only observed withn-type Schottky diodes. The sensitivity difference is the standardattributed to two effects. First, the mfp electrons and copper films hasbeen measured to be half that of mfp and silver films. Second, theinterface properties of silver/silicon and copper/silicon are verydifferent, e.g., the copper reacts with silicon and may form a suicidewhile similar reactions are not known for silver on silicon. Since thediodes are annealed, copper/silicon interfaces are expected to berougher and have more scattering centers than silver/silicon interfaces.The enhanced roughness may reduce the transmission probabilityconsiderably, in agreement with reported results on mfp in silicideswhich are smaller than in metals.

The p-type silver/silicon diodes seen in FIG. 2(a) is approximately 3.5times less sensitive than n-type device. These might be explained bydifferences in the mfp path of holes and electrons in silver, asobserved previously in gold and in platinum silicon thin films. In theseprior art observations, the attenuation lengths of holes were a factorof approximately 1.5 smaller than for electrons. Additionally,sensitivity differences may be related to the energy spectra of holesand electrons excited by the surface reactions. The d-bands of bulksilver cannot contribute to the ballistic current, since they are morethan 2.7 electron volts below the Fermi energy. The ballistic chargecarriers thus have nearly a free SP character. The probability ofexciting an electron-hole pairs is assumed to depend on the jointdensity of states of occupied and empty electronic states. Since thedensity of states of silver increases slightly with energy in the rangeof ±3 electron volts around the Fermi energy, electrons closer to theFermi energy are excited more effectively. Consequently, the energydistributions of ballistic holes and electrons are not symmetric aroundthe Fermi energy and on the average the ballistic electrons are expectedto have higher kinetic energies than hot holes. Such an asymmetry wouldlead to a significant sensitivity difference between p- and n-typediodes.

FIG. 3 is a graph of the chemicurrent as a function of time for atomichydrogen and deuterium reacting with a 75 angstrom silver film onn-silicon (111). The oscillations in the decay curve for deuterium aredue to plasma fluctuations. Although for the exposure graph of FIG. 3the impingement rate of atomic deuterium was approximately twice aslarge as that for atomic hydrogen, the measured chemicurrent withdeuterium exposure was smaller by a factor 3, i.e., a sensitivity toatomic deuterium is six times smaller than that to atomic hydrogen. Theslight differences in the strengths of hydrogen and deuterium metalbonds cannot explain this observed isotope effect. A reduced adsorptionprobability for deuterium on silver would also not account for thisobservation, since this would affect the decay rate as well. The decayrates in FIG. 3 differ by a factor of approximately 1.8 which may beexclusively attributed to the flux difference between hydrogen anddeuterium. The isotope effect implies different velocities andinteraction times of the incoming hydrogen and deuterium by a factor ofv². The interaction time, however, is still in the 10⁻¹³ second rangewhich is at least an order of magnitude longer than time constants ofelectron transfer between the substrate and the impinging atoms. For thesame reason, we exclude internal exoelectron emissions which requiresquenching of resonant charge transfer into the affinity level of theapproaching atom accompanied by a drastic change of the surfaceoxidation state.

It is believed that the more relevant mechanism behind the isotopeeffect is likely to be the de-excitation of highly excited vibrationalstates formed under chemisorption. The transition probability betweentwo vibrational levels in an nonharmonic potential decreases the largerthe difference of the two respective quantum numbers. Hence,de-excitation most likely occurs in multiple steps. The spacing betweenthe vibrational levels, ie., the density of states of vibrationalstates, determines the released energy in each step, and the states inthe enharmonic deuterium-silver potential are closer to each other thanfor the hydrogen-silver bond. Since the formation energies ofdeuterium-silver and hydrogen-silver bonds are almost identical, thedeuterium-silver vibrational energy may be relaxed in more steps ofsmaller energy quanta compared to the hydrogen-silver case. This wouldresult in ballistic charge carriers of lower energies and explain thesmaller sensitivity to deuterium.

In summary, the foregoing disclosure is the first direct detectionreported of hot electrons and holes excited by adsorption of atomichydrogen deuterium on ultrathin silver and copper films as achemicurrent. The current is measured in the large-area Schottky diodeformed from these metals on oriented silicon (111). The devices areunique sensors that can discriminate atomic from molecular hydrogen aswell as deuterium from hydrogen atoms. The chemicurrents decayexponentially with exposure time and reach a steady-state value. Thisbehavior corresponds to occupation of free adsorption sites by hydrogenatoms and a balance between adsorption and abstraction. The currents aresmaller if p-type semiconductors are used and if the devices are exposedto deuterium rather than hydrogen. This isotope effect opens a new wayof monitoring reactions on metal surfaces and will certainly initiatefurther investigations to clarify the mechanism of the excitation. Wehave developed a reliable device structure for the fabrication ofultrathin Schottky diodes.

Many alterations and modifications may be made by those having ordinaryskill in the art without departing from the spirit and scope of theinvention. Therefore, it must be understood that the illustratedembodiment has been set forth only for the purposes of example and thatit should not be taken as limiting the invention which could be morebroadly or narrowly defined later by patent claims.

For example, it is expected that the chemoelectric phenomena associatedwith atomic and molecular interactions at metal surfaces will be foundto show that chemical reactions at metal surfaces can directly transferreaction energy to electrons in the metal. The phenomena can thus beutilized as the basis of a new class of solid state sensors. Theadsorption induced current of different transition metal-semiconductorcombinations will provide a means of systematically varying therelationships between the adsorbate and the metal surface and theelectronic environment in the metal at the metal-semiconductorinterface, and within the semiconductor. New sensor structures will haveimproved device sensitivity and allow discrimination of the electronenergy with operation at room temperature and above. Bimolecular surfacecatalyzed reactions in addition to chemisorption is usable for directexcitation of charge carriers during formation of bonds between surfaceadsorbed species. In addition to the sensor performance and sensitivityfor detection of hydrogen, several important adsorbates are possibleexpressly including CO, CO₂, O₂(O), N₂(N), NO, C₂H₂, C₂H₄, and C₂H₆.FIG. 4 shows the chemisorption current as a function of time for CO witha 60 Angstrom Ag/n-silicon (111) sensor of the invention at 135K. FIG. 5shows the chemisorption current as a function of time for CO with an 80Angstrom Ag/n-silicon (111) sensor of the invention at 135K. FIG. 8shows the response to molecular oxygen. Each adsorbate will have aunique current intensity and rate of signal decay which will allowdifferentiation of adsorbates.

The sensor response to surface catalyzed reactions of severalcombinations of these species following absorption are with the scope ofthe invention expressly including the reactions of CO+O₂, CO+NO, andH₂+O₂. In the sensor 40 of FIG. 6 a catalytic layer 46 is added on topof metal layer 44 disposed on doped silicon layer 42 fabricated in amanner consistent with the teachings of the invention. The chemisorptioncurrent is measured by an integrating voltage amplifier 48. Catalyticlayer 46 is chosen specifically to catalyze a selected reaction whichthen directly interacts with metal layer 44 to create a measurablechemicurrent. As shown diagrammatically in FIG. 7 a plurality of sensors40 of the type shown in FIG. 6 can then be combined in an array, eachone of which plurality of sensors 40 has a different catalytic layer 46to detect a corresponding plurality of different adsorbates through xand y-addressing circuits 50 and current detector 52. In this manner anelectronic nose is realized.

The words used in this specification to describe the invention and itsvarious embodiments are to be understood not only in the sense of theircommonly defined meanings, but to include by special definition in thisspecification structure, material or acts beyond the scope of thecommonly defined meanings. Thus if an element can be understood in thecontext of this specification as including more than one meaning, thenits use in later in a claim must be understood as being generic to allpossible meanings supported by the specification and by the word itself.

The definitions of the words or elements of the following claims are,therefore, defined in this specification to include not only thecombination of elements which are literally set forth, but allequivalent structure, material or acts for performing substantially thesame function in substantially the same way to obtain substantially thesame result. In this sense it is therefore contemplated that anequivalent substitution of two or more elements may be made for any oneof the elements in later defined claims or that a single element may besubstituted for two or more elements in later defined claims.

Insubstantial changes from the claimed subject matter as viewed by aperson with ordinary skill in the art, now known or later devised, areexpressly contemplated as being equivalently within the scope of theinvention. Therefore, obvious substitutions now or later known to onewith ordinary skill in the art are defined to be within the scope of thedefined elements.

The invention is thus to be understood to include what is specificallyillustrated and described above, what is conceptionally equivalent, whatcan be obviously substituted and also what essentially incorporates theessential idea of the invention.

1-21. (canceled)
 22. A sensor comprising: a semiconductor having a topand a bottom; a metal film deposited on said top of said semiconductorwherein the interface between said metal film and the top of saidsemiconductor forms a Schottky Barrier; at least one oxide layer locatedon said top of said semiconductor; at least one metal pad formed on saidoxide layer; said metal film connected to said at least one metal pad;an oxide layer located on said bottom of said semiconductor; and a backohmic contact formed on said oxide layer located on said bottom of saidsemiconductor.
 23. The sensor of claim 22 wherein said metal film isdeposited on at least one inclination in said one oxide layer on saidsemiconductor before being connected to said metal pad.
 24. The sensorof claim 22 further comprising a catalyst layer located on top of saidmetal film.
 25. The sensor of claim 22 further comprising an array ofdifferent metal films located on an array of different semiconductors.26. The sensor of claim 22 wherein said sensor is used to detect achemical located on top of said metal film adjacent to saidsemiconductor.
 27. The sensor of claim 22 wherein said sensor is used todetect a chemical reaction on top of said metal film adjacent to saidsemiconductor.
 28. The sensor of claim 22 wherein said sensor is used todetect a chemical or chemical reaction on top of said metal filmadjacent to said semiconductor wherein the presence of the chemical orchemical reaction causes a chemically induced electron-hole productionat the Schottky Barrier that creates a measurable chemicurrent.
 29. Thesensor of claim 22 wherein said sensor uses non-adiabatic energydissipation during adsorption to detect the presence of a chemical or achemical reaction on top of said metal film adjacent to saidsemiconductor.